Latino/Hispanic youth have higher rates of substance use at earlier ages compared to youth of other ethnocultural backgrounds (Johnston et al., 2008), and Mexican-origin youth may be at particular risk for substance use. Acculturation is an important risk factor for substance use in youth of Mexican-origin, such that increased orientation to American culture is related to increased substance use likelihood. In addition, increased orientation to Mexican culture may protect youth from problematic substance use (Allen et al., 2008; Gil, Wagner, & Vega, 2000). Understanding how cultural orientation influences substance use is paramount for developing effective prevention and intervention strategies for Mexican origin youth, with the goal of reducing disparities in early substance use rates. Theory suggests that acculturation is a process (i.e., acculturation levels change over time), yet there is a dearth of research on acculturation change over time, and on the variability of acculturation change among youth. Moreover, only one study has examined whether youth substance use changes as a function of changes in acculturation. One mechanism that appears to partially explain the link between youth acculturation and substance use is disrupted family dynamics, including increased family conflict (Samaniego & Gonzales, 1999). Family acculturation dynamics, and resulting negative family processes, also appear to be key pathway for youth substance use intentions (Martinez, 2006) and behavior (Unger, Ritt-Olsen, Soto, Baezconde-Garbanati, 2009). However, evidence for the mechanism of disrupted family dynamics is provided solely by cross-sectional studies, and there are no longitudinal studies regarding concurrent changes in acculturation, family dysfunction, and youth substance use. For these reasons, this project aims to examine acculturation over time as it relates to increases in substance use, and concurrent increases in negative family processes. Specifically, I propose to examine growth curve models of acculturation to examine the relationship between changes in cultural orientation and changes in substance use likelihood, and to examine concurrent family disruption as a factor that may partially explain the link between acculturation change and later substance use and intentions. The proposed study will use data from 640 Mexican-origin youth and their parents across four time points. Data includes measures of acculturation (youth and parent report), substance use (youth report), and family dynamics (including youth and parent self-report and observer ratings) assessed across four years, from 5th to 8th grade.